1. Field of the Invention
The present invention relates to a display device, and more particularly to a display device having a front light.
2. Description of the Background Art
In recent years, liquid crystal display devices, having desirable characteristics such as a small thickness and a small power consumption, have been widely used in various applications, including OA equipment such as word processors and personal computers, PDAs (personal digital assistance) such as electronic organizers, and camcorders with liquid crystal monitors.
Unlike a self-luminous display device such as a CRT (Cathode Ray Tube), a PDP (Plasma Display Panel) and an EL (Electro Luminescence) device, a non-luminous type display device such as a liquid crystal display device does not emit light by itself, and displays characters and images by controlling the amount of externally-provided light to be transmitted or reflected.
These liquid crystal display devices are generally classified into those of transmission type and those of reflection type.
A transmission type liquid crystal display device displays an image by modulating light from an illuminator (so-called xe2x80x9cbacklightxe2x80x9d) arranged on the rear side of a liquid crystal display element through a liquid crystal layer of the liquid crystal display element.
On the other hand, a reflection type liquid crystal display device displays an image by using ambient light, and thus does not require a backlight. Therefore, a reflection type liquid crystal display device has a number of advantages such as a small weight, a small thickness, and a low power consumption. Moreover, the visibility does not lower even under very bright environments, and a displayed image can be viewed more clearly under bright environments. However, with a reflection type liquid crystal display device, the display brightness and the contrast ratio are substantially influenced by the conditions under which it is used, e.g., the brightness of the environment. Particularly, the visibility lowers significantly under dark environments.
Therefore, some reflection type liquid crystal display devices are provided with an illuminator for improving the display quality in cases where ambient light of a sufficient intensity is not available. The illuminator is provided on the front side of the liquid crystal display element, and is called xe2x80x9cfrontlightxe2x80x9d as opposed to the illuminator of a transmission type liquid crystal display device being called xe2x80x9cbacklightxe2x80x9d.
FIG. 7 schematically illustrates a reflection type liquid crystal display device 600A, which is disclosed in xe2x80x9cMonthly DISPLAYxe2x80x9d, November 2001 issue (Techno Times Co., Ltd.), p. 56 to p. 62, xe2x80x9cFrontlight Technical Trendxe2x80x9d.
As illustrated in FIG. 7, the reflection type liquid crystal display device 600A includes a reflection type liquid crystal display element 620, and a front light 610 provided on the viewer side of the reflection type liquid crystal display element 620.
The front light 610 includes a light source 611 and a light guide 613, and a reflection film 612 is provided so as to surround the light source 611.
The light guide 613 includes a light-receiving surface 613a for receiving light from the light source 611, a light-exiting surface 613b provided so as to be substantially perpendicular to the light-receiving surface 613a, and a counter surface 613c opposing the light-exiting surface 613b. The reflection type liquid crystal display element 620 is provided on one side of the light guide 613 that is closer to the light-exiting surface 613b. The counter surface 613c includes a propagation surface 613c1 and a reflection surface 613c2, and has a saw-toothed cross section.
Light emitted from the light source 611 enters the light guide 613 through the light-receiving surface 613a, and propagates therethrough while being totally reflected between the light-exiting surface 613b and the propagation surface 613c1. A portion of the light, which is being propagated through the light guide 613, is reflected by the reflection surface 613c2 and emitted toward the reflection type liquid crystal display element 620 through the light-exiting surface 613b. 
The reflection type liquid crystal display element 620 includes a pair of glass substrates 621a and 621b, and a liquid crystal layer 622 and a reflector 623 provided between the pair of glass substrates 621a and 621b. A polarizer 624a and a xcex/4 film 624b are provided on the glass substrate 621a on the viewer side (the side closer to the front light 610).
In the reflection type liquid crystal display device 600A, circularly-polarized light that has entered the liquid crystal layer 622 via the polarizer 624a and the xcex/4 film 624b passes through the liquid crystal layer 622, is reflected by the reflector 623, and passes again through the liquid crystal layer 622. In this process, the amount of light is controlled as the polarization thereof is modulated by the liquid crystal layer 622, thereby displaying an image.
Anti-reflection films 631a and 631b are provided, through a vapor deposition process, on the light-exiting surface 613b of the light guide 613 and the polarizer 624a of the reflection type liquid crystal display element 620, respectively, for an anti-air-interface-reflection treatment. Therefore, the air-interface reflection is suppressed at the interface between the light-exiting surface 613b and the air, and at the interface between the polarizer 624a and the air, thereby suppressing a decrease in the contrast ratio due to light being reflected at the interfaces toward the viewer side.
Moreover, the article (xe2x80x9cMonthly Displayxe2x80x9d, November 2001 issue (Techno Times Co., Ltd.), p. 56 to p. 62, xe2x80x9cFrontlight Technical Trendxe2x80x9d) also discloses a reflection type liquid crystal display device 600B illustrated in FIG. 8.
In the reflection type liquid crystal display device 600B, the polarizer 624a and the xcex/4 film 624b are attached to the light-exiting surface 613b of the light guide 613 via the adhesive layer 631, as illustrated in FIG. 8, and light exiting through the light-exiting surface 613b of the light guide 613 passes through the polarizer 624a and the xcex/4 film 624b via the adhesive layer 631 to become circularly-polarized light. The light reflected at the interface between the xcex/4 film 624b and the air, and at the interface between the glass substrate 621a on the viewer side of the reflection type liquid crystal display element 620 and the air, passes again through the xcex/4 film 624b and is then absorbed by the polarizer 624a, thereby suppressing a decrease in the contrast ratio due to the reflection of light at the interfaces.
Moreover, Japanese Laid-Open Patent Publication No. 11-149252 discloses a reflection type liquid crystal display device 600C in which the light guide 613 and the reflection type liquid crystal display element 620 are attached to each other via the adhesive layer 631, as illustrated in FIG. 9.
In the reflection type liquid crystal display device 600C, the light guide 613 and the reflection type liquid crystal display element 620 are attached to each other via an adhesive layer 632, as illustrated in FIG. 9, there is no air-element interface between the light guide 613 and the reflection type liquid crystal display element 620. Therefore, an unnecessary air-interface reflection does not occur, thereby realizing a display with a high contrast ratio.
However, an in-depth research by the present inventor has revealed that the reflection type liquid crystal display devices 600A, 600B and 600C have the following problems.
First, as illustrated in FIG. 7, with the anti-air-interface-reflection treatment using anti-reflection films, the air-interface reflection cannot be suppressed sufficiently. For example, in a case where an interference film made of a transparent material is formed as an anti-reflection film, the air-interface reflection can be reduced only to about 0.1 to 0.3% even if five or six interference films are laminated together. In view of this, in the reflection type liquid crystal display device 600A, the decrease in the contrast ratio cannot be suppressed sufficiently. Moreover, the anti-air-interface-reflection treatment as described above has some dependency on the wavelength of light, and cannot effectively reduce the air-interface reflection across the entire visible spectrum. Therefore, coloring of light may occur due to the air-interface reflection in a particular wavelength range, thereby lowering the display quality.
As illustrated in FIG. 8, the effect of suppressing the air-interface reflection obtained by attaching the polarizer 624a and the xcex/4 film 624b on the light guide 613 is significantly greater than that obtained through the anti-air-interface-reflection treatment using anti-reflection films. Therefore, the reflection type liquid crystal display device 600B provides a significantly improved the contrast ratio. However, since the light-exiting surface 613b of the light guide 613 is adjacent to the adhesive layer 631, but not to the air layer, total reflection does not occur efficiently at the light-exiting surface 613b. Specifically, after the light, which is being propagated through the light guide 613, is reflected by the propagation surface 613c1 and reaches the light-exiting surface 613b, the light is not totally reflected by the light-exiting surface 613b and a portion of the light enters the polarizer 624a via the adhesive layer 631, whereby the light is partially absorbed. Therefore, the amount of light propagating through the light guide 613 decreases, and the amount of light that is reflected by the reflection surface 613c2 of the counter surface 613c to be emitted through the light-exiting surface 613b also decreases, thereby lowering the display brightness.
In order to suppress the decrease in the brightness, Japanese Laid-Open Patent Publication No. 2000-155315 discloses a method in which the refractive index of the adhesive layer 631, which is provided between the light-exiting surface 613b of the light guide 613 and the polarizer 624a is set to be different from that of the light guide 613 (for example, the refractive index of the adhesive layer 631 is set to be smaller than that of the light guide 613), thereby causing total reflection to occur efficiently at the light-exiting surface 613b, and thus improving the brightness.
However, when the refractive index of the adhesive layer 631 is set to be different from that of the light guide 613, an interface reflection occurs at the interface between the light guide 613 and the adhesive layer 631 or at the interface between the adhesive layer 631 and the polarizer 624a due to the refractive index differences therebetween, thereby decreasing the contrast ratio. Thus, in the reflection type liquid crystal display device 600B, it is difficult to improve both the brightness and the contrast ratio, irrespective of the setting of the refractive index of the adhesive layer 631.
Also in the reflection type liquid crystal display device 600C illustrated in FIG. 9, since the light-exiting surface 613b of the light guide 613 is adjacent to the adhesive layer 632, but not to the air layer, total reflection does not occur efficiently at the light-exiting surface 613b, thereby decreasing the display brightness.
In order to suppress the decrease in the brightness, Japanese Laid-Open Patent Publication No. 11-219610, Japanese Laid-Open Patent Publication No. 11-326903 and Japanese Laid-Open Patent Publication No. 11-339528 disclose a method in which the refractive index of the adhesive layer 632, which is provided between the light-exiting surface 613b of the light guide 613 and the reflection type liquid crystal display element 620, is set to be different from that of the light guide 613 (for example, the refractive index of the adhesive layer 632 is set to be smaller than that of the light guide 613), thereby causing total reflection to occur efficiently at the light-exiting surface 613b, and thus improving the brightness.
However, when the refractive index of the adhesive layer 632 is set to be different from that of the light guide 613, an interface reflection occurs at the interface between the light guide 613 and the adhesive layer 632 or at the interface between the adhesive layer 632 and the reflection type liquid crystal display element 620 due to the refractive index differences therebetween, thereby decreasing the contrast ratio. Thus, in the reflection type liquid crystal display device 600C, it is difficult to improve both the brightness and the contrast ratio, irrespective of the setting of the refractive index of the adhesive layer 632.
As described above, a display device providing both a desirable brightness and a desirable contrast ratio has not yet been realized in the art.
The present invention has been made in view of the problems as set forth above, and has an object to provide a display device capable of displaying an image with a high brightness and a high contrast ratio.
A display device of the present invention includes: a light source; a light guide having a light-receiving surface for receiving light from the light source and a light-exiting surface for emitting the light, which has entered the light guide through the light-receiving surface; a reflection type display element provided on one side of the light guide that is closer to the light-exiting surface; and a refractive-index-adjusted layer provided between the light guide and the reflection type display element and having a different refractive index from that of the light guide, wherein: the light guide and the reflection type display element are attached to each other via the refractive-index-adjusted layer; and at least one of an interface between the light guide and the refractive-index-adjusted layer and an interface between the refractive-index-adjusted layer and the reflection type display element is subjected to an anti-interface-reflection treatment for suppressing a reflection of light at the interface. Thus, the object set forth above is achieved.
The anti-interface-reflection treatment may be performed by forming an interference film made of a transparent material between the light guide and the refractive-index-adjusted layer and/or between the refractive-index-adjusted layer and the reflection type display element.
It is preferred that the interface between the light guide and the refractive-index-adjusted layer and the interface between the refractive-index-adjusted layer and the reflection type display element are both subjected to the anti-interface-reflection treatment.
The refractive-index-adjusted layer may function as an adhesive layer for attaching the light guide and the reflection type display element to each other.
Another display device of the present invention includes: a light source; a light guide having a light-receiving surface for receiving light from the light source and a light-exiting surface for emitting the light, which has entered the light guide through the light-receiving surface; a reflection type display element provided on one side of the light guide that is closer to the light-exiting surface; a polarizer provided between the light guide and the reflection type display element; and a refractive-index-adjusted layer provided between the light guide and the polarizer and having a different refractive index from that of the light guide, wherein: the light guide and the polarizer are attached to each other via the refractive-index-adjusted layer; and at least one of an interface between the light guide and the refractive-index-adjusted layer and an interface between the refractive-index-adjusted layer and the polarizer is subjected to an anti-interface-reflection treatment for suppressing a reflection of light at the interface. Thus, the object set forth above is achieved.
The anti-interface-reflection treatment may be performed by forming an interference film made of a transparent material between the light guide and the refractive-index-adjusted layer and/or between the refractive-index-adjusted layer and the polarizer.
It is preferred that the interface between the light guide and the refractive-index-adjusted layer and the interface between the refractive-index-adjusted layer and the polarizer are both subjected to the anti-interface-reflection treatment.
The refractive-index-adjusted layer may function as an adhesive layer for attaching the light guide and the polarizer to each other.
It is preferred that the refractive index of the refractive-index-adjusted layer is smaller than that of the light guide.
It is preferred that a value of the refractive index of the refractive-index-adjusted layer is greater than 1 and less than or equal to 1.4.
The functions of the present invention will now be described.
In a display device of the present invention, the refractive-index-adjusted layer having a different refractive index from that of the light guide is provided between the light guide and the reflection type display element, whereby the total reflection of light occurs efficiently at the light-exiting surface of the light guide. Therefore, the decrease in the amount of light being propagated through the light guide is suppressed, thereby realizing a bright display. Moreover, at least one of the interface between the light guide and the refractive-index-adjusted layer and the interface between the refractive-index-adjusted layer and the reflection type display element is subjected to the anti-interface-reflection treatment for suppressing the reflection of light at the interfaces. Thus, the reflection of light is suppressed at the interfaces, thus realizing a display with a high contrast ratio.
For example, the anti-interface-reflection treatment is performed by forming an interference film made of a transparent material between the light guide and the refractive-index-adjusted layer and/or between the refractive-index-adjusted layer and the reflection type display element.
If the interface between the light guide and the refractive-index-adjusted layer and the interface between the refractive-index-adjusted layer and the reflection type display element are both subjected to the anti-interface-reflection treatment, it is possible to further improve the contrast ratio.
In another display device of the present invention, the refractive-index-adjusted layer having a different refractive index from that of the light guide is provided between the light guide and the polarizer, whereby the total reflection of light occurs efficiently at the light-exiting surface of the light guide. Therefore, the decrease in the amount of light being propagated through the light guide is suppressed, thereby realizing a bright display. Moreover, at least one of the interface between the light guide and the refractive-index-adjusted layer and the interface between the refractive-index-adjusted layer and the polarizer is subjected to the anti-interface-reflection treatment for suppressing the reflection of light at the interfaces. Thus, the reflection of light is suppressed at the interfaces, thus realizing a display with a high contrast ratio.
For example, the anti-interface-reflection treatment is performed by forming an interference film made of a transparent material between the light guide and the refractive-index-adjusted layer and/or between the refractive-index-adjusted layer and the polarizer.
If the interface between the light guide and the refractive-index-adjusted layer and the interface between the refractive-index-adjusted layer and the polarizer are both subjected to the anti-interface-reflection treatment it is possible to further improve the contrast ratio.
In order for the total reflection of light to occur efficiently at the light-exiting surface of the light guide, the refractive index of the refractive-index-adjusted layer is preferably smaller than that of the light guide. If the value of the refractive index of the refractive-index-adjusted layer is greater than 1 and less than or equal to 1.4, a sufficient brightness can be obtained.
The production of the display device is facilitated if the refractive-index-adjusted layer functions also as an adhesive layer for attaching the light guide and the reflection type display element to each other, or the light guide and the polarizer to each other.
The present invention provides a display device capable of displaying an image with a high brightness and a high contrast ratio.
In a display device of the present invention, the refractive-index-adjusted layer having a different refractive index from that of the light guide is provided between the light guide and the reflection type display element, whereby the total reflection of light occurs efficiently at the light-exiting surface of the light guide. Therefore, the decrease in the amount of light being propagated through the light guide is suppressed, thereby realizing a bright display. Moreover, at least one of the interface between the light guide and the refractive-index-adjusted layer and the interface between the refractive-index-adjusted layer and the reflection type display element is subjected to the anti-interface-reflection treatment for suppressing the reflection of light at the interfaces. Thus, the reflection of light is suppressed at the interfaces, thus realizing a display with a high contrast ratio.
In another display device of the present invention, the refractive-index-adjusted layer having a different refractive index from that of the light guide is provided between the light guide and the polarizer, whereby the total reflection of light occurs efficiently at the light-exiting surface of the light guide. Therefore, the decrease in the amount of light being propagated through the light guide is suppressed, thereby realizing a bright display. Moreover, at least one of the interface between the light guide and the refractive-index-adjusted layer and the interface between the refractive-index-adjusted layer and the polarizer is subjected to the anti-interface-reflection treatment for suppressing the reflection of light at the interfaces. Thus, the reflection of light is suppressed at the interfaces, thus realizing a display with a high contrast ratio.